1. Field
Disclosed herein are tires for passenger cars and more particularly to the treads with which the said tires are provided, and more specifically still relates to the tires intended to be mounted on high-performance vehicles.
2. Description of Related Art
It is known practice to form tread patterns in such a way as to give the tread a directional nature. A directional tire comprises a tread that has a directional pattern such that when the tire turns in the preferred direction, this tire offers better grip than the grip offered when it is turning in the other direction. What is meant by tread pattern is a plurality of cuts made in the said tread in an appropriate pattern, the said cuts having widths and depths which are likewise appropriate. What is meant by cuts is either grooves or incisions, the latter (the incisions) differing from the former in terms of the widths (distances between opposing faces) which are generally smaller than 2 mm and more appropriately still, smaller than 1 mm).
A common directional tread pattern has main grooves which are arranged in such a way as to constitute a series of V-shaped grooves all pointing in the same direction right around the tread of the tire. Usually, for maximum grip during acceleration of high-performance vehicles, the V-shaped grooves point in the direction of travel such that the median part (the part close to the median plane of the tire) of each V-shaped groove comes first in the imprint that the tire leaves on the ground.
Examples of this kind of tire are given in European Patents 064934, 0721853, 1238827 or U.S. Pat. No. 4,057,089.
While these patterns do offer a certain advantage when driving on a very wet road surface, it has been found that this type of pattern may present disadvantages during road holding tests on dry road surfaces when the vehicle is being tested at the limits of grip, particularly under cornering. What actually happens is that the curve of transverse thrust as a function of the slip angle of such a tire has a characteristic appearance demonstrating an increase in the said transverse thrust up to a maximum value, followed by an appreciable reduction in transverse thrust for high slip angles (an example of such a curve is shown on the curve identified C0 in FIG. 2 of this document).
Under strong acceleration, and for certain specific maneuvers such as “overturning” or turning at the limit of grip for lateral accelerations in excess of 0.8 times the acceleration due to gravity, this may result in an impairment of the equilibrium of the vehicle fitted with tires provided with such patterns.
Patent document EP0721853 discloses a variant of such a pattern in which a plurality of additional grooves have been introduced. Some of these additional grooves are formed in such a way as to connect pairs of main grooves situated on one and the same side of the tread with respect to the median plane. Other additional grooves are formed to connect pairs of main grooves situated one on each side of the median plane. What is meant by a groove connected to another groove is that an additional groove opens at its ends into main grooves.
These additional grooves here are directed in such a way as to be substantially perpendicular to the main grooves that they connect. Between two main grooves, a first additional groove is formed between the axially inner end of another main groove and a second main groove so as to be substantially perpendicular to the second main groove. This first additional groove makes an angle of close to 45 degrees with the circumferential direction. A second additional groove is formed axially on the outside of this first additional groove. In this prior art, the main grooves and the first additional grooves delimit a plurality of elements that are discontinuous in the circumferential direction, some of the corners of which are subjected to high raised pressures during turning maneuvers (that is to say notably under high transverse acceleration). The additional grooves of this prior art are not good from a transverse rigidity standpoint because when this tread is subjected to a transverse load, notably during a turning maneuver, an appreciable loss of rigidity is observed, giving rise to a reduction in the cornering thrust.